Follow-up to f5ca96528ddc6cc14e5c86cf2172418ab863e845
Why this change?
`RSpec.current_example.metadata[:extra_failure_lines]` can be `nil` and
calling `<<` on `nil` is not a good idea.
What does this change do?
Set `RSpec.current_example.metadata[:extra_failure_lines]` to `""` as
long as there are exceptions.